1. Field of the Invention
This invention relates generally to printed circuit boards, and pertains more particularly to a method for trimming the leads from such boards.
2. Description of the Prior Art
Generally, printed circuit boards have various electrical components located on one side of the substrate laminate with wiring on the other side. Stated somewhat differently, the leads of the various components are initially inserted through holes in the laminate from the unwired side and are then soldered in place, usually by dip soldering or flow soldering. The soldering not only anchors the leads in place but also effects the necessary electrical connections between the components and the printed wiring.
Since the leads on the various electrical components vary in length, it follows that the leads when inserted will project various distances from the back side of the substrate laminate, usually beyond the solder fillet resulting from the soldering operation. Hence, the need exists for trimming all of the leads so that their ends reside in a single or common plane. Frequently, the cutting or trimming is not only through the leads themselves but also through some of the solder fillets surrounding the leads, the frequency depending in large measure as to how short the ends are to be cropped. This results in a number of problems, the problems varying with how the leads are actually cut.
One common procedure for trimming printed circuit boards is to employ a cutting tool utilizing a cutter ring having a smooth beveled cutting edge. The cutting tool is either hand held or mounted in a fixture (usually the latter) so that the ring can be advanced, while rotating, through the projecting end portions of the leads and also through any solder encircling the leads at that level.
An example of the unserrated type of ring cutter is illustrated in U.S. Pat. No. 2,894,583, granted on July 14, 1959 to Dagfin Johnstad, and titled "Honeycomb Cutter". While the performance of a cutting of this character is generally acceptable, there still are a number of shortcomings for which satisfactory solutions have not been presented.
One difficulty stemming from the use of a smooth edge cutter ring is that while each resharpening or regrinding is less costly than some arrangements, a considerable amount of heat is generated during use because of the friction that is developed from the rubbing that takes place. The rubbing that occurs causes the ring to wear rapidly, thereby requiring a greater number of resharpenings. Also, the cutter ring as it is advanced, tends to load up with both solder and metal from the lead being cut. Of course, the thicker the leads and/or the thicker the solder fillets, the greater the loading rate.
Additionally, so-called foldovers and flags are produced which result in a defective circuit board if these partially severed lead sections are not removed, their removal being time-consuming and likely to damage the rather delicate printed wiring if insufficient care is not exercised.
Still further, a smooth or unserrated ring normally requires that some medium be employed for holding the leads in a rigid or semirigid fashion during the cutting action. Otherwise, they may flex to such a degree that the cutter ring cannot cut effectively. Solder, wax, paraffin, vacuum packaging and the like have been resorted to, all of which contribute to the overall fabrication expense.
In some respects, an improvement upon the smooth or unserrated variety of cutting ring referred to above is a cutter ring utilizing a serrated cutting edge. Although not described as being useful for trimming printed circuit boards, an example of a serrated ring is set forth in U.S. Pat. No. 2,531,841, issued on Nov. 28, 1950 to Frank J. Cashin. This style of cutter ring makes use of numerous serrations that are evenly spaced with respect to each other around its peripheral cutting edge.
However, with the cutter ring rotating at a constant speed, objectionable vibrations are set up in the trimming of printed circuit boards. Unfortunately, vibrations can be sufficiently severe so as to damage the circuit components in a number of instances. The vibrations also cause the substrate laminate to vibrate toward and away from the cutting edge of the cutter ring. This results in excessive rubbing against the tool's cutting edge, the resulting friction creating a concomitant amount of heat that has proved even more objectionable than when using smooth cutting edges. The serrations themselves are difficult to sharpen and cause the uncut or partially cut leads to be pushed over by reason of the blunting or dulling that results and which is accelerated by reason of the excessive friction. Consequently, this technique also causes objectionable foldovers and flags to be produced. The employment of uniformly spaced serrations produces an extreme shocking action on the material being cut and also on the serrated ring. This at times proves detrimental to the substance being cut and can cause fracturing and dulling of the ring.
Because of the above, manufacturers of equipment designated for trimming printed circuit boards endeavor to reduce the problems caused by vibration by going to very high rotational speeds. While this does tend to reduce the vibration on the material being cut, it nontheless magnifies or aggravates an already unsafe situation as far as the worker is concerned, for it is he who is vulnerable to injury from the flying pieces of a broken cutter ring.